When thermally cutting a metal sheet material (hereinafter referred to as “workpiece”) by use of a conventional thermal cutting machine such as a plasma cutting machine or laser cutting machine, molten material produced from cuts of the workpiece by a plasma arc or laser beam flies in all directions in the form of spatter (molten metal splash) owing to the gas pressure of plasma gas jetted from the torch. The spatter as well as fumes (metal vapor) and noise which are generated simultaneously with the spatter worsen the working condition of the work site. As attempts to solve this problem, there have been developed various techniques for preventing dispersion of spatter generated in the course of cutting operations and for shutting off fumes, arc light and noise.
One known technique for preventing dispersion of spatter and for shutting off arc light is such that a fire-resistant fabric such as asbestos is fixedly disposed to surround the torch like a curtain, for preventing damage caused by spatter (a first known technique).
Another proposal is the hood set out in Japanese Patent Publication KOKAI Gazette No. 7-136767 (a second known technique). A cross-sectional view of the hood of this publication is shown in FIG. 9.
Referring to FIG. 9, a hood B is mounted at the leading end of a plasma torch body 101 so as to be movable in an axial direction of the plasma torch body 101. A hood holder 103 is secured to the leading end of the plasma torch body 101 for restricting the movement of the hood B and holding the hood B at a specified restricted position. The hood B includes a mounting portion which is positioned at the substantial center of the hood B and has a hole corresponding to the outside diameter of the plasma torch A, and a wall portion surrounding the mounting portion, extending downwardly therefrom. The mounting portion and the wall portion are made from a sound absorbing material. A cap 102 for the plasma torch A is covered with a sound insulating wall 104 made from a steel sheet, stainless steel sheet or synthetic resin sheet having fire resistance. The hood B is lowered by its own weight relative to the plasma torch body 101 to the aforesaid restricted position where the hood B is held by the hood holder 103 so that the hood B and the workpiece C has a specified positional relationship even when the plasma torch A is lowered below a specified level.
According to the above publication, if there exists a clearance between the hood B and the workpiece C, spatter, noise and light will leak therethrough and therefore the hood B and the workpiece C should be in close contact with each other. The above publication also discusses that if the hood B and the workpiece C closely contact each other, one end of a cut of the workpiece C may uprise (see FIG. 7) or the surface of the workpiece C, in which a thermal strain has occurred, may collide with the hood B with the result that processing of the workpiece C cannot be continued, and that, in view of this, it is desirable to use a steel sheet or stainless steel sheet having moderate flexibility or a flame-resistant synthetic resin sheet as the construction material of the hood B.
Another proposal is a dust collector for a thermal cutting machine such as disclosed is Japanese Patent Publication Kokai Gazette No. 9-57438 (a third known technique). This dust collector covers the thermal cutting machine and the entire upper face of the main body table, and includes a slide cover extensible in a longitudinal direction of the main body table. The slide cover is single-hinged with at least one longitudinal end being fixed. For replacement of the workpiece, the torch can be housed at the side of the fixed end of the slide cover. Dust on the upper and lower parts of the main body table can be collected outside the machine by means of a dust collector pipeline. The effect of the dust collector resides in that since cutting operation can be carried out with the slide cover portion being in its closed state, entirely covering the cutting machine, dust produced above the main body table is prevented from dispersing within the work site by the slide cover portion and noise, fumes, spatter, arc light and others can be shut off during the cutting operation.
The first to third techniques described earlier, however, present the following disadvantages.
In the first known technique, although a hood made from a fire-resistant fabric such as asbestos encloses the torch in a curtain-like manner, this fabric hood is melted down within a short time by the spatter (i.e., splash) of high-temperature molten metal so that it loses its original function and, thus, has very short service life. Therefore, even if the manufacturer fits the machine with a new hood before delivery to the user, frequent replacement of the hood and troublesome maintenance are required at the users site. In the present circumstances, the hood which has lost its shielding function is sometimes used without replacement.
The following is the drawbacks of the second known technique disclosed in Japanese Patent Publication Kokai Gazette No. 7-136767.
(1) During an actual plasma cutting operation, there may occur such a phenomenon that as shown in FIG. 7, a small cut piece of the workpiece, which has been cut on the table, topples as it cannot keep a horizontal state, losing a support from the work-supporting bars of the table. Since the hood B surrounding the torch horizontally moves with its lower end being close to the workpiece, there is the possibility that slight inclination of the workpiece may cause interference between the lower part of the hood B and the raised portion of the workpiece, leading to breakage of the hood B. Further, in some cases, the interference between the lower part of the hood B and the workpiece imposes an excessive load on the horizontal movement of the torch, bringing the machine to a stop.
(2) Although the publication proposes, as a measure for solving the above problem, use of a steel sheet or stainless steel sheet having moderate flexibility or use of a flame-resistant synthetic resin sheet as the construction material of the hood B, the wall portion of the hood B, in reality, bends to a considerable extent when it interferes with the workpiece C. Further, it is undeniable in consideration of the frequency of the interference that the interference could be an obstacle to the movement of the torch.
In this case, it is necessary, on account of damage to the hood, to stop the machine to avoid the interference with the workpiece. If the interference results in breakage of the hood, there will arise a need for work such as restoration, which leads to interruption of the automatic operation of the machine. Further, the down time and recovery time cause a drop in the machine utilization rate and, in consequence, decreased productivity, so that this technique is off from practical use because it cannot meet the demands toward unattended operation such as nighttime automatic operation. In addition, for applying this previous technique to an actual plasma cutting machine, the following conditions should be met: the workpiece has to be cut into shapes which do not cause an uprise of a cut piece and the degree of thermal deformation of the workpiece must be insignificant. In this respect, the second known technique is difficult to put to practical use.
(3) As seen from FIG. 4, a largest amount of spatter is produced, dispersing in all directions in the piercing phase prior to the cutting phase. The direction in which a largest amount of spatter disperses is a horizontal direction along the workpiece. Taking this into account, it is necessary to bring the lower end of the hood into sufficiently tight contact with the workpiece in order to effectively shut off the spatter. The use of a steel sheet, stainless steel sheet or flame-resistant synthetic resin sheet as the material of the hood, which is proposed by the second known technique, provides a service life longer than heat-resistant fabrics but cannot maintain a little clearance between the workpiece and the lower end of the hood, because such materials are melted down sooner or later by a large amount of spatter produced during the piercing phase. Therefore, the effect of shutting off spatter does not last for a long time. For this reason, the second known technique has not proved practical.
(4) The second technique also proposes use of a transparent synthetic resin sheet for the side walls of the hood to enable easy supervision of the condition of the plasma arc and the condition of the workpiece being processed. However, spatter adheres to and melts the synthetic resin sheet, so that the synthetic resin sheet loses transparency and, therefore, cannot come in practical use.
(5) In addition, it is necessary to remove the cap of the torch to replace consumable parts attached to the leading end of the torch such as the nozzle and electrodes several times per day, although the frequency of replacement depends on the amount of cutting. If there is a hood or cover near the torch, workability at the time of replacement decreases. Although the second known technique teaches that replacement of the consumable parts can be carried out while lifting up the hood by hand, this causes a considerable decrease in workability because the removal of the cap and the replacement of the consumable parts need to be carried out by one hand, while the other hand is tied up with pressing-up of the hood.
The third known technique disclosed in Japanese Patent Publication Kokai Gazette No. 9-57438 provides the slide cover which is designed to cover the thermal cutting machine and the entire upper face of the main body table and is therefore large and heavy, so that opening and closing of the slide cover requires a lot of labor and accompanies many troubles. Since the slide cover needs to be frequently opened and closed, for instance, at the time of setting and removal of a workpiece, replacement of the nozzle of the torch, and monitoring of the cut condition of the workpiece, down time due to the opening/closing of the slide cover increases, causing a decrease in machine utilization rate, and thus, the third technique presents a problem in productivity. The slide cover is difficult to manufacture because it is a large-sized component made by sheet metal processing. In addition, the clearance in the sliding part of the slide cover requires high dimensional accuracy in order to provide smooth opening and closing movement and prevent a leakage of fumes to the outside of the cover. These factors entail high production cost.
The invention has been directed to overcoming the foregoing problems and a primary object of the invention is therefore to provide a thermal cutting machine and a cutting method using the same, which employ a simple, long-life torch hood for effectively shutting off spatter generated in a thermal cutting operation to keep good working conditions in the work site, the torch hood enabling easy supervision of the cut condition of the workpiece and easy replacement of the consumable parts of the torch.